1. Field of the Invention
The invention pertains to the field of digital imaging and, more particularly, to white balance control for adjustment of color or shade under variable lighting conditions.
2. Discussion of the Related Art
Many digital cameras and imaging devices use an array of charge-coupled device (CCD) detectors to capture the image. Each CCD detector accumulates a charge when exposed to light. The magnitude of charge represents the intensity of the light to which the CCD detector has been exposed. Each CCD detector is coupled to an analog to digital converter (A/D) that produces a digital signal representing the charge and intensity of light received by the CCD detector. Color images are captured by placing a color filter before each CCD detector. For example, a grouping of CCD color detectors may include two green detectors, a red detector and a blue detector. The color composition of an image is captured by taking the digital value of each detector according to color type. By way of example, U.S. Pat. No. 5,995,142 issued to Matsufune shows circuitry for capturing color images where analog signals from the CCD detectors are sorted into color channels, subjected to analog processing via a variable gain amplifier at the channel level, and converted to digital signals.
Complimentary metal oxide semiconductor (CMOS) detectors may be used in place of CCD detectors. CMOS detectors differ from CCD detectors in that CMOS detectors provide a continuous signal representing light intensity, whereas CCD detectors are integrating devices. Collectively, CCD detectors, CMOS detectors, and other useful forms of photodetectors or photodiodes are referred to herein as “detectors.
It is problematic in the art of digital photography that the digital cameras are used under variable light conditions. The signal received at each CCD detector may vary, for example, depending upon the type of light source even when the total intensity of light between two different sources is the same. These variations occur because different light sources differ in their emission spectra. As compared to natural sunlight, the emissions from a tungsten source contain more red light than blue or green light. If a digital camera is tuned or calibrated for natural sunlight, the resulting image under a tungsten source has an undesirable reddish/orange appearance. In addition, even ambient light from the same type of source may vary in intensity and composition, for example, as natural sunlight that is filtered by clouds or by atmosphere according to the season of the year. These problems are mitigated by providing digital cameras with white balance adjustments.
White balance is achieved when the image of a neutral white object produces equalized or normalized signals in each of the red, blue and green CCD detector channels. A digital camera that is tuned or calibrated to achieve white balance under one set of illumination conditions is not necessarily calibrated for a different set of illumination conditions. As shown in the '142 patent to Matsufune, one method for overcoming this problem is to alter the gain of the variable gain amplifiers according to principles of calibration involving the adjustment of variable gain coefficients. The '142 patent shows a system for scanning an image according to a calibration algorithm that follows a blackbody radiation pattern. The blackbody radiation pattern is used to adjust or select the scan area that is used for white balance adjustment purposes, in combination with stored standardized calibration data representing the color white.
Yet another white balance adjustment technique is described in U.S. Pat. No. 6,411,331 issued to Sansom-Wai et al. Circuitry is coupled to a CCD array for generating uncorrected digital image data including a chromaticity value and a luminance value for each of a plurality of pixels. A control circuit maps a two dimensional representation of the uncorrected image to select a spatial region that is used for calibration purposes in making the white balance adjustment.
U.S. Pat. No. 6,249,323 issued to Van Der Voort describes non-scanning circuitry that sets variable gain coefficients by equalizing chromiluminsence signals to achieve white balance. The variable gain coefficients may be manually set through adjustment of a variable resistor, or they may be set automatically in response to input signals, e.g., from an imaged calibration target of the color white or a specified shade of gray. Gain correction is performed according to a mathematical algorithm that exponentially and proportionally relate the input signals to stored reference signals for the calibration target.
U.S. Pat. No. 6,038,339 issued to Hubel et al. describes a variety of mathematical techniques that may be used to calculate white balance and describes improvements that pertain to correcting color of the image based on the illumination type. A correlation matrix memory or associative matrix memory is used to achieve identical results to the known color-in-perspective method. The methods are improved by adding Bayesian or other correlation statistics. The correlation matrix memory is built to correlate the data from any picture image to reference images under a range of illumination type. When a camera, scanner, or the like, produces a picture image, the data is converted to chromaticity and a vector is created corresponding to the values existing in the scene. This vector is multiplied by each column in the correlation matrix to establish a new matrix. Each column is then summed, and the resulting values form a vector that is compared to stored reference information representing the likelihood of each reference source being the illumination type for the scene. The vector values can be density plotted where each value is plotted at the chromaticity of the illumination for that particular column. From this plot normal statistical methods are used to estimate the likely illumination type of the scene. Greatly reduced computational requirements result because simple binary matrix algorithms replace complex geometrical calculations.
None of the foregoing references address a more practical problem that is commonly encountered by photographers. The quality of photographic images is enhanced by calibrating digital cameras under actual conditions of illumination encountered as the photographs are being taken. Calibration of this type requires that the photographer have access to a calibration target. The calibration target is frequently specified as white paper having 18% gray. The target is subject to degradation under conditions of actual use, such as contamination with grime, creasing, or tearing. Nothing in the art addresses the practical problem of having to produce a pristine calibration target in the field.